1. Field of the Invention
The present invention relates to an interrogator, transponder, and transponder identification method, to identify the transponder in communication between the interrogator and the transponder.
2. Description of the Related Art
Identification (ID identification) processing to identify transponders is executed in a system such as a RFID (Radio Frequency Identification) system or an IC (Integrated Circuit) card system, in which data is transmitted/received via wireless communication between plural transponders (a transponder, tag, IC card, etc.) and interrogators (an interrogator, reader/writer, R/W, etc.)
If plural transponders are given with respect to one interrogator, all the transponders respond at once together to an identification command from the interrogator. This results in a problem that radio signals collide with each other so that the interrogator is unable to receive any response.
International standard ISO-18000 part 6 is an identification scheme with anti-collision.
Of the ISO-18000 part 6, Type A is a random number time division scheme which is called a slot ALOHA method. Operation of the Type A will be described below. In response to an identification command from an interrogator, a transponder generates a random number, and sends an ID to a slot corresponding to the value. If the ID is received successfully, the interrogator notifies the transponder of a receipt response signature. If the transponder fails to receive the signature (i.e., a collision is detected), the transponder generates a random number again and sends the ID after waiting a time corresponding to the value.
Meanwhile, Type B of the ISO-18000 part 6 is a random number state transition scheme. Operation of the Type B will be described below. Each transponder has a timer counter and responds to an identification command from an interrogator only when the timer counter counts zero. If a interrogator fails to receive the response (i.e., a collision is detected), the interrogator notifies corresponding transponders of this failure. Those transponders that have responded just before and have received the acknowledgement of this failure generate a random number (“0” or “1”). Of the transponders, those that have generated “0” send the ID again while the others that have generated “1” let the timer counters count up.
An address bit transfer scheme is a kind of identification processing scheme with ant-collision noted above (for example, see U.S. Pat. No. 6,727,803). Response devices send IDs by use of slots which deal with IDs in units of bits. A interrogator receives one or both of “0” and “1”. If the interrogator receives one of “0” and “1”, the interrogator requests transfer of a next bit from the transponder which sends the received one of “0” and “1”. Otherwise, if the interrogator receives both of “0” and “1”, for example, the interrogator prioritizes “1” and requests transfer of a next bit from the transponder which sends the received “1”. In addition, the interrogator stacks the received “0” on a bit string in which previously received bits are arrayed.
However, in the random number time division scheme and the random number state transition scheme, the transponders decide whether a response is possible or not according to the random numbers. Therefore, if a large number of transponders are provided, the collision rate rises. Meanwhile, in the slot ALOHA method, if only a small number of transponders are provided, the response period is fixed depending on the number of slots, resulting in a problem of low efficiency.
The address bit transfer scheme noted above is an efficient search scheme which avoids collision by use of uniqueness of IDs. According to this scheme, however, ID lengths have to be searched for regardless of the number of transponders. A problem hence arises in that the identification processing takes a long time. For example, if two transponders are provided, ID lengths such as 96 bits, 128 bits, and the like have to be searched for.